1. Field of the Invention
The present invention is directed to the recovery of minerals such as phosphate and, more particularly, to a column flotation cell and related method to enhance phosphate recovery.
2. Description of Related Art
Presently, phosphate is recovered from a sand-clay mixture that is mined from mineral deposits. Traditionally, phosphate is mixed with a collector, suspended in water and urged to the surface of an aeration tank called a flotation cell.
For example, as shown in FIG. 1 herein, U.S. Pat. No. 4,735,709 describes a flotation system 10 with means for introducing a gaseous medium such as air, to facilitate flotation. The system 10 generally includes a flotation vessel 12, and two air sparger systems 14 and 16 for introducing a gaseous medium or air into the vessel 12.
The vessel 12 is formed as an upright elongated cylinder having a vertical wall 18 and a bottom wall 20. The vessel 12 is typically open at an upper end 22. A substantially horizontally-disposed constriction plate 24 is located within the vessel 12, spaced above the bottom wall 20, to separate the vessel 12 into a flotation compartment 26 above the constriction plate 24 and a distribution compartment 28 below the constriction plate 24. The constriction plate 24 has a plurality of orifices 30 to permit passage of aerated water from the distribution compartment 28 to the flotation compartment 26.
A feed well 32 is supported within the upper end 22 of the flotation compartment 26 by a base 33. A feed tube 34 from an external source of aqueous slurry (not shown) delivers a controlled quantity of the aqueous slurry to the feed well 32. The feed well 32 has an overflow baffle 36 to distribute the aqueous slurry throughout the flotation compartment 26. The feed well of this conventional design is located under the water level 11 in the vessel 12.
Air bubbles are introduced into the bottom of the fluid vessel 12 by flowing aerated water through the air sparger 14 and into a manifold 38 exiting into the vessel 12 via the orifices 30. The air bubbles aerate the slurry in the vessel 12.
The slurry entering vessel 12 contains phosphate, impurities and a collector. The use and types of collectors are well known in the art. An example of a typical collector used in the art is a hydrocarbon such as tall oil. See, e.g., U.S. Pat. No. 6,178,383. The phosphate suspended in the aqueous slurry adheres to the rising air bubbles and collects at the upper end of the flotation compartment 26 as a froth.
A launder 44 is provided at the upper end 22 of the vessel 12, atop the cylinder wall 18. The launder 44 generally includes a circular inner wall 45, a relatively higher outer wall 47 and a bottom wall 49 that form a trough 51 to receive the froth, which overflows from the flotation compartment 26. The froth overflows into the trough 51 when the froth inside the flotation compartment 26 rises and spills over the top of the lower circular inner wall 45. An outlet 46 is provided in the outer wall 47, near the bottom wall 49, to convey the overflowing phosphate-laden froth from the launder 44 to further processing or storage.
The impurities including sand and clay contained within the slurry along with any residual phosphate that is not captured by the levitating air bubbles percolates downwardly through the aqueous slurry by gravity. An opening 48 is formed through the center of the constriction plate 24 into which the impurities pass through. An outlet 50 extends from the opening 48 through the bottom wall 20 of the cylinder 12. The outlet 50 allows removal of the impurities from the vessel 12.
The orifices 30 can “choke” over a period of time because the velocity of the air bubbles moving through the orifices 30 is not high enough to prevent the downwardly percolating impurities including sand from plugging the orifices 30. The result of the choking is that aerated water will not be able to enter and circulate through the vessel 12, which results in poor separation of the phosphate from the impurities.
Further, the system 10 generally exists an as alkaline environment, which can allow algae growth. Algae growth is promoted near the orifices 30 because the levitating air bubbles create low-turbulence areas near the orifices 30. Algae will attach and grow at these low-turbulence areas such that over time the orifices 30 will get sealed off, preventing the even dispersion of aerated water throughout the vessel 12.
U.S. Pat. No. 4,735,709 also describes the use of a separate air sparger system 16 that discharges aerated water above the constriction plate 24 into the vessel 12 via orifices 41 in pipes 40. However, these orifices 41 can also choke over a period of time as the impurities from the slurry percolate downward in the vessel 12 for the same reasons as described above.
The choking of the orifices 30 and 41 can not only prevent the even aeration of the vessel 12, but also require maintenance involving the cleaning or redrilling of the orifices 30 and 41 in order to un-choke or un-plug them. The phosphate separation process, therefore, has to be suspended for maintenance and cannot be carried on as a continuous process. A continual need for maintenance introduces down time and maintenance costs into the separation process, which results in reduced recovery of phosphate and high cost of operation.
As also known in the art, the above-described column flotation cell can require significant capital expenditures to build, depending upon the size, component parts, etc. The system is also known to require a significant amount of energy to thrust aerated water from the bottom to the top of the column flotation cell.
Thus, although the prior art described above has generally been widely used for the purposes of recovering minerals such as phosphate from impurities, it still does not disclose or teach a column flotation cell and a method of use that reduces capital costs, lowers energy consumption, prevents substantial choking of the column and allows easier maintenance.